The exemplary embodiment relates to registration of images in printing systems. It finds particular application in connection with a registration system for a multicolor printing system which compensates for fluctuations in the position of an image receiving surface between marking stations.
To provide accurate printing of images, multicolor digital marking systems need to maintain adequate color to color registration. In systems that utilize an elongate image receiving surface, such as a paper web or a belt, the receiving surface reaches a first marking station where a marking material of a first color is applied to the surface, e.g., by firing ink jets, exposing an image on a photoconductive material, or applying toner particles to a selectively imaged photoconductive member. The receiving surface then moves on to a second marking station, where an image or marking material of a second color is applied, and so forth, depending on the number of colors. The timing of the actuation of the second marking station can be controlled as a function of the speed of the image receiving surface so that the images applied by the two marking stations are registered one on top of the other to form a composite, multicolor image. A high degree of process direction alignment can be achieved by implementing what is generally known as reflex printing, where the speed or position of the image receiving surface is measured with an encoder at a certain location and then the images are timed accordingly. For example, an encoder is associated with a drive nip roller. The rotational speed of the roller is used to calculate the speed of the image receiving surface passing through the nip. The time for actuating the first, second, and subsequent marking stations is then calculated, based on their respective distances from the drive nip roller and the determined speed of the image receiving surface.
In the case of an electrophotographic printer, an encoder may be placed on the photoreceptor belt to measure the exact speed of the belt at each instant of time. The timing from this signal can then be used to time the firing of the laser raster output scanner (ROS) or light emitting diode (LED) bar so that an even spacing of lines is imaged on the photoreceptor, thus compensating for any variability in the photoreceptor speed from a set speed. In a multicolor system, the timing from the encoder can also be used to determine the exact time to fire successive color images to obtain good color on color registration, again compensating for any photoreceptor speed variations.
An implicit assumption of reflex printing systems is that the belt or web is infinitely stiff (i.e., it does not stretch or change length) such that the encoder measurement of the web or belt velocity enables an exact prediction of correct registration. In situations where the belt or web exhibits any sizeable amount of stretch or deformation, reflex printing techniques may still be subject to misregistration errors.
Image production systems often use a multitude of imagers in different locations along an image or paper transport path (e.g. belt loop, web . . . ). Each imager generates a separation, i.e. part of the total image (e.g. a particular color) at a particular location. The motion quality of the transport system between the imager locations determines the alignment of the separations (e.g. color registration) and the quality of the resulting image. Reflex printing measures the velocity of the image transport system and adjusts the imager timing to make the separations coincide. Double reflex printing measures the velocity of the image transport system in two different locations (e.g. before and after the imaging stations) to compensate for tension variation. Disadvantages of this method are: 1) this second velocity measurement is an additional expense, and 2) in many cases, the velocity measurement device measures the angular velocity of a somewhat compliant (e.g. rubber coated) drive roll that propels the image transport system. This measurement is known to be inaccurate which leads to a reduced quality of the produced image.